Mechanical pencils having an automatic lead feed arrangement are commonly utilized in automatic drawing or plotting machines. Such mechanical pencils must be capable of feeding the lead segments so that the pencil constantly draws without gaps and without requiring attention from the operator.
A variety of designs for mechanical pencils capable of providing automatic feed have been proposed. Examples of these are shown in U.S. Pat. Nos. 3,998,558, 4,015,269, 4,149,813, 4,172,673, 4,230,413, 4,269,524, 4,346,392, and 4,490,061. The patent to Katz, 4,490,061, shows a mechanical pencil that advances the lead segments as a result of the alternating steps of pressing the pencil tip against the paper and lifting the pencil from the paper. Mechanical pencils which operate in such a manner are thus well adapted to use in automatic plotting machines in which the pencils are repeatedly brought into and out of contact with the surface during the normal drawing operations.
While automatic feeding mechanical pencils suitable for use with plotting machines are known, they typically have been complex in structure and consequently somewhat expensive and prone to mechanical failure. Generally, the more complex the feeding mechanism, the more subject the mechanism is to fouling with dirt or graphite debris and the more critical the requirements for proper alignment of the lead segments to enable correct feeding of the segments in sequence.